1. Field of the Invention
The present invention is directed to an x-ray apparatus of the type having an x-ray source and an x-ray receiver, the x-ray source being adjustable relative to a subject and emitting x-rays in a direction toward the x-ray receiver during the course of radiological exposures of the subject, and having an arrangement in the beam path of the x-rays for influencing the shape and/or the intensity profile of the x-ray. The invention is also directed to a method for influencing x-rays in the beam path.
2. Description of the Prior Art
When producing x-ray images of a subject, there is a basic desire to present the region of the subject of interest for the examination with very good quality in the x-ray image, i.e. with little image noise. The subject under examination is therefore charged with such an x-ray dose that a good signal-to-noise ratio is established, i.e. the image noise is relatively low in the image region representing the region of interest. Particularly in the medical application of x-rays, for example the production of x-ray images of relatively small tissue regions of diagnostic interest such as the heart or vessels, it is disadvantageous that the tissue that surrounds the tissue of interest and is diagnostically less relevant or irrelevant and the presentation of which with poorer quality could be tolerated is likewise exposed to the high x-ray dose in the image registration.
In order to reduce the radiation stress for the tissue surrounding the tissue of interest, for example, it is known from computed tomography to employ wedge filters that usually influence an x-ray beam emanating from an x-ray source such that the intensity of the x-rays of the x-ray beam incident on the central x-ray detectors of the x-ray receiver is higher than the intensity of the x-rays that strike the outer x-ray detectors of the x-ray receiver, since these x-rays are already attenuated by the wedge filter before passing through the subject. When, accordingly, the tissue of interest of a subject under examination is placed in the rotational center of the computed tomography apparatus, a comparatively low radiation stress occurs for the tissue surrounding the tissue of interest as a result of the effect of the wedge filter. The wedge filters employed are usually adapted to the absorption profile of a homogeneous, circular phantom. The intensity profile of the x-rays that can be generated with such a wedge filter, accordingly, is relatively well-adapted to the absorption profile of the phantom, but is only well-adapted to the absorption profile of different body slices of a patient or different patients in exceptional cases.
Another procedure for reducing the radiation stress on the tissue surrounding the tissue of interest is to employ a collimator allocated to the x-ray source having a central radiation window that is smaller in size, so that, for example given x-ray imaging with a computed tomography apparatus, the tissue of interest is imaged in a circular slice whose center coincides with the rotational center of the computed tomography apparatus. This procedure, however, allows only the diameter of the circular slice exhibiting the tissue of interest to be set, and also requires that the tissue of interest in an examination be placed as exactly as possible in the rotational center of the x-ray system.
An object of the present invention is to provide an x-ray system of the type initially described wherein the generation of a qualitatively high-grade x-ray image of a region of interest of a subject is simplified and involves less radiation stress for the region surrounding the region of interest of the subject. It is also based on an object of the present invention to provide a method for reducing the radiation stress in imaging with x-rays for a region surrounding a region of interest in a subject.
The object is inventively achieved by an x-ray apparatus having an x-ray source and an x-ray receiver, the x-ray source being adjustable relative to a subject and emitting x-rays in the direction toward the x-ray receiver during the course of radiological exposures of the subject, and having an arrangement in the beam path of the x-rays for influencing the shape and/or the intensity profile of the x-ray, wherein the arrangement can be dynamically set for influencing the shape and/or the intensity profile of the x-rays during radiological exposures of the subject. Such dynamic, i.e. automatically variable, setting of the arrangement allows the shape and/or the intensity profile of the x-rays emanating from the x-ray source in the direction of the subject to be readily adapted in the context various, successive radiological exposures of the subject, for example due to adjustments of the x-ray source relative to the subject under examination which result in different positions of the focus of the x-ray source relative to a region of interest in the subject, so that the region of the subject surrounding the region of interest is charged with no radiation or with only slight x-radiation. Given a rotational adjustment of the x-ray source relative to the subject, the region of interest thereby need not necessarily be located in the rotational center of the x-ray source, so that the image registration is in turn simplified due to the elimination of tedious alignment processes of the subject and of the x-ray system relative to one another. The adjustments of the arrangement for influencing the shape and/or the intensity profile of the x-radiation dependent on the respective conditions for different radiological exposures of a subject preferably ensues by a control unit allocated to the arrangement.
In an embodiment of the invention the arrangement for influencing the shape and/or the intensity profile of the x-rays is a collimator allocated to the x-ray source and exhibiting a radiation window, with the size of the radiation window being dynamically adjustable. In a version of this embodiment, the size of the radiation window of the collimator can be varied by elements of the collimator that are movable relative to one another. By an appropriate adjustment of the elements, accordingly, not only the shape of the x-ray beam, i.e. its beam cross-section, but also the position of the radiation window of the collimator can be set relative to the focus of the x-ray source, and thus relative to the path of the x-rays. In this way, the path of the x-rays can be influenced such that, independently of the position of the focus relative to a subject under examination, essentially only the region of interest of the subject is transirradiated. The x-rays that strike the material of the collimator are significantly attenuated in comparison the x-rays passing through the radiation window of the collimator that can be dynamically adjusted in terms of its size, shape and position, are even completely absorbed by the material of the collimator, so that the x-ray dose with which the region of a subject surrounding a region of interest is charged is significantly lower than the x-ray dose that charges the region of interest.
In a version of the this embodiment that the elements of the collimator move along a circular path whose center of curvature preferably lies in the focus of the x-ray source. This version is advantageous because, given a constant thickness of the material of the adjustable elements of the collimator, the possibilities of variation arising from the adjustment of the elements in the absorption profile of the collimator are minimized since the x-rays travel essentially the same distance when traversing the elements. By providing elements that are nearly circularly curved to emulate the curvature of the circular path, this allows, for selected settings of the elements of the collimator and intensities of the x-rays, determination and storage of only a few absorption profiles of the collimator required for the imaging. These profiles need only be subjected to simple shift operations for changing (adjusting) the absorption profile of the collimator corresponding to the actual setting of the elements given an x-ray exposure. Planar or differently shaped elements can be employed instead of circularly curved elements, but these should be moved along the circular path given settings of the elements modifying the size of the radiation window for minimizing the variation possibilities in the absorption profile of the collimator. Otherwise, the absorption profiles of the collimator vary greatly given different settings of the elements, since the path length of the x-rays through the collimator material or the adjustable elements thereof is a function of the distance of the elements from the focus of the x-ray source. In this case, respective absorption tables must be produced for various settings of the elements, since the absorption profiles required for the reconstruction of x-ray images cannot be determined with simple shift operations of a few, identified absorption profiles.
In an embodiment of the invention the arrangement for influencing the shape and/or the intensity profile of the x-rays include at least one wedge filter allocated to the x-ray source and having elements that are movable relative to one another, these elements in one version of the invention moving on a circular path like the elements of the collimator for minimizing the variation possibilities in the absorption profile of the wedge filter. The shape and/or the intensity profile of the x-rays emanating from the x-ray source also can be influenced with the adjustable elements of the wedge filter so that a diagnostically less relevant region surrounding a region of diagnostic interest is charged with a comparatively low x-ray dose.
A further embodiment of the invention has a measuring arrangement for acquiring the setting of the arrangement for influencing the shape and/or the intensity profile of the x-rays. In another embodiment of the invention, the arrangement for influencing the shape and/or the intensity profile of the x-rays can effect a significant, detectable boost in the intensity profile of the x-rays. In a version of this embodiment, the significant detectable change in the intensity profile of the x-rays is produced by thinning the material and/or thickening the material of arrangement for influencing the shape and/or the intensity profile of the x-rays. Such a thinning of material can, for example, be in the form of a slot, and the thickening of material can be in the form of a ridge. The measuring arrangement as well as the structure that effects the detectable change in the intensity profile serve the purposexe2x80x94in every x-ray exposure for the current setting of the arrangement for influencing the shape and/or the intensity profile of the x-radiationxe2x80x94of determining the absorption profile that is required for the reconstruction of images from the absorption tables that have been (preferably) determined preceding subject measurements and have been stored.
The inventive method for influencing the shape and/or the intensity profile of the x-ray beam with an inventively fashioned x-ray apparatus includes setting of the beam influencing arrangement on the basis of a function determined before a subject measurement (exposure) dependent on the positions to be assumed by the x-ray source during the subject measurement. The setting of the arrangement preferably ensues with a control that sets the arrangement for influencing the shape and/or the intensity profile during the subject measurement according to the function determined offline, i.e. before a subject measurement. For a computed tomography apparatus, the setting of the arrangement and thus the modulation of the x-rays is, for example, a function of the rotational angle of the x-ray source around the rotational center of the computed tomography apparatus.
The object of the invention is also achieved in a method for influencing the shape and/or the intensity profile of the x-rays of an inventively fashioned x-ray apparatus, wherein the influencing of the shape and/or of the intensity profile of the x-rays ensues on the basis of radiation attenuation values determined during a subject measurement, i.e. online. In this case, the radiation attenuation values determined with a data measuring system are made available to the control unit that calculates and initiates the setting of the arrangement for influencing the shape and/or the intensity profile on the basis of the radiation attenuation values.